I find this to be a really interesting concept because people often have such incorrect ideas about it. I have often heard people say, "you're not really sick...it's psychosomatic." Or when I suggest that some problem they're having is psychosomatic they respond by saying, "Oh no, the pain (or whatever the symptoms are) are real." These responses imply that psychosomatic means fake and that there are no real symptoms or illnesses. However, a psychosomatic illness or symptom is indeed real...very real, but is brought on by mental processes. In other words, a psychosomatic illness is an illness that is brought on by the mind and not from a virus, bacteria, injury, etc.
For example, many people experience stomach pains when they are dealing with a stressful mental situation (e.g., getting all worked up before a really important test). These stomach pains are absolutely real, as are the chemical reactions occurring in the body as a result of the person's thought process (thinking about the test). But, the stomach pains are not caused by some stomach infection or air born virus, but rather the process of thinking about the stressful event which in turn triggers all sorts of chemical and biological events to happen and eventually cause the stomach pain. Remember, psychosomatic pains, illnesses, etc. are very real, but are brought on by thought or the mind rather than bacteria, injury, etc.